This invention relates generally to an improved ballast circuitry for metal halide arc discharge lamps enabling sustained operation from a typical 120 volt 60 Hz AC power source and, more particularly, to simplified operating circuit means for so doing.
Conventional metal halide type arc discharge lamp operation can be characterized as successively requiring (1) an initial high voltage breakdown mode, (2) a glow-to-arc transition mode, and (3) a steady state run mode. The already known overall ballast circuitry includes means to provide typical 2-4 kilovolts to achieve initial breakdown, and means which then must continue to provide sufficient "open circuit voltage" (OCV) to exceed that of the lamp, backed up with sufficient current to cause the lamp to proceed into the arc condition. Once the arc tube in such AC operated lamps is in the arc mode, the arc must reignite at the beginning of each half-cycle of the AC power supply. During the warm-up process the voltage required to reignite commonly employed lamps on each half-cycle rises for a period of time until the arc stablizes. If the current flowing in the arc tube drops below a critical value, the arc condition may extinguish causing the arc tube to revert to its glow-to-arc mode or even the initial breakdown mode. The reestablishment of the desired arc condition may therefor require high voltage for breakdown and a source of glow-to-arc transition energy on an immediate cycle by cycle basis. As a result, such AC operated metal halide arc discharge lamps are operated from a supply voltage in the range 240-277 volts which is high enough to provide the OCV and meet the reignition requirements. In such conditions, the starting circuit portion of the overall circuitry need only provide high voltage sufficient for breakdown in the arc tube. The power line is higher than the lamp terminal voltage (215 volts typical OCV) enough of each half-cycle to cause sufficient current to flow in the lamp run circuitry and establish the required arc, limited by the impedance of a single inductor.
When it is desired to operate such a ballasted lamp with a typical 120 volt rms AC power supply, however, the line voltage does not meet the minimum OCV and some other means must be provided to get through the glow-to-arc transition. Once in the run mode, the lamp terminal voltage becomes much lower than the supplied line voltage so the lamp is able to operate thereafter from such 120 volt AC power supply in series with the suitable current-limiting means. In a DC circuit, the required OCV can be obtained from a relatively simple voltage doubler, but for an AC circuit, some other solution must be provided. For example, the series inductor can be in the form of a step-up auto transformer but this is bulky as well as heavy and wasteful in terms of additional energy dissipation from this circuit component. Typically such circuit component remains in the ballast circuitry during lamp operation but the step-up part is not really needed once the transition to the run mode is made.
Already known ballast circuit means enabling sustained 120 volt AC operation of the arc discharge lamp but which further employs an incandescent lamp filament is disclosed in U.S. Pat. No. 4,555,647, filed in the name of the present inventor and others. As therein described, the operating circuitry discharges energy stored in a multiple capacitor arrangement as the voltage applied to the arc tube transitions to its low value and supplies or controls the necessary energy applied to the arc tube to prevent the extinction of the arc condition in the arc tube during the steady state run mode of operation. The ballast circuitry controlling the steady state run mode of operation of the arc tube is effectively in parallel with the arc tube and incandescent filament arrangement until the ballast circuit is rendered conductive at which time the ballast circuit discharges, in a serial manner, the energy stored in its capacitor devices into the arc tube and filament arrangement. The specific circuit arrangement described for doing so employs starting circuit means being operated in combination with a pair of multicomponent biasing circuit networks for the required switching action. The auxiliary circuit network said therein to be required for such cooperation with the starting means includes (a) capacitive energy storage means comprising a first capacitor and a second capacitor each provided with means connected to one end thereof for respectively charging each of said first and second capacitors during a preselected portion of the said applied AC voltage and with each of said first and second capacitors having the other end thereof connected respectively to an opposite input terminal of said operating circuit, (b) switching means comprising first and second current control devices each having a first, a second and a third terminal, (c) a first bias network and a second bias network respectively connected to said first terminal of each of said first and second current control device, said first and second bias networks being respectively responsive to a selected portion of the cycle of said AC voltage effective to respectively render said first and second current control devices conductive, and (d) said first and second current control devices each respectively having a second terminal connected to opposite terminals of said output stage, and each of said first and second current control devices having its third terminal respectively connected to said first end of said first and second capacitors effective to respectively discharge said first and second capacitors across the gas discharge tube when said first and second current control devices are rendered conductive.
It remains desirable to provide still more effective and simpler ballasting circuitry for sustained operation of a metal halide type arc discharge lamp from a 120 volt AC power supply.
Accordingly, a novel operating circuit arrangement for sustained lamp operation has now been discovered which continuously maintains the desired run mode condition in the arc tube being employed with a minimum number of required circuit components.
Another object of the present invention is to provide operating circuitry for a metal halide arc discharge lamp which operates in a novel manner also effecting energy conservation.
Still another object of the present invention is to provide operating circuitry for a metal halide arc discharge lamp wherein a pair of circuit components enables relaxation type starting circuit means to provide the glow-to-arc transition in the lamp being operated, then automatically switches to a different mode to supply reignition energy when needed in the arc tube, and finally becomes inactive in the circuit arrangement once the run mode has been reestablished, thereby dissipating no further energy.
These and other objects of the present invention, both as to circuit construction and method of operation, may best be understood by reference to the following description taken in conjunction with the accompanying drawing